The production of heat-insulating bodies (vacuum panels) from a porous material and a gas-tight film covering it all over, with the "hollow space" formed by the porous material being evacuated, is known. The porous material may consist either of a porous bulk material or else of a rigid foam. The film usually consists of combinations of several layers, with various objects, such as that of providing a diffusion barrier to different gases and giving mechanical strength, being fulfilled by the different layers.
Most marketable vacuum panels have a core of porous bulk material, usually based on silicates.
In EP-B 188 806 rigid polyurethane-based (PU) foams are described as filling material for vacuum panels. The problem with this process is that of achieving the required open-cell structure by precise selection of formulation and by maintaining particular processing variables, since PU rigid foams are predominantly closed-celled. Another problem with the use of PU rigid foams as supporting material for vacuum panels is the evolution of volatile constituents such as amines, blowing agents and possibly also monomers.
An advantage of rigid PU foams as compared with porous bulk materials, on the other hand, is their exact dimensioning as the finished vacuum panel, as well as their exact dimensional accuracy.
Surprisingly, it has now been found that an excellent supporting material for vacuum panels can be economically produced by mixing with binders the plastics foam powder or plastics foam flakes, which arise, for example, during the recycling of waste foams, preferably from refrigerator insulation, and processing the mixture with the use of pressure and optionally temperature to form moulded products having a bulk density of from 50 to 350 kg/m.sup.3, preferably of from 100 to 250 kg/m.sup.3. Here moulded products, for example slabs, having a fine pore structure and high strength and dimensional accuracy are obtained.